Implement bytemuck traits where appropriate

[eira-fransham]

A nice optimisation provided by bytemuck is the ability to do zeroed heap allocations using calloc safely. Since the default parking_lot Mutex and RwLock have an all-zeroes default state, then this means for any safely zeroable type T, RwLock<T> and Mutex<T> are also safely zeroable - and, by extension, FairMutex too. ReentrantMutex is not, however. Implementing these traits - even if behind a feature gate - would make some use-cases using collections of mutexes faster.